Reactive-Transport Simulation ofPhosphorus
in the Sewage Plume at theMassachusetts
Military Reservation,Cape
Cod, Massachusetts

By David L. Parkhurst, Kenneth G. Stollenwerk,
and John A. Colman

Water-Resources Investigations Report 03-4017

ABSTRACT

The subsurface transport of phosphorus introduced
by the disposal of treated sewageeffluent to
ground-infiltration disposal beds atthe Massachusetts
Military Reservation onwestern Cape Cod was
simulated with a three-dimensionalreactive-transport
model. Thesimulations were used to estimate
the load of phosphorus transported to Ashumet Pond duringoperation
of the sewage-treatment plant—from1936 to 1995—and
for 60 years followingcessation of sewage disposal.
The modelaccounted for spatial and temporal
changes inwater discharge from the sewage-treatment
plant,ground-water flow, transport of associatedchemical constituents, and a set of chemicalreactions, including phosphorus sorption onaquifer materials, dissolution and precipitation ofiron- and manganese-oxyhydroxide and iron phosphate
minerals, organic carbon sorption anddecomposition,
cation sorption, and irreversibledenitrification.The flow and transport in the aquifer were simulated
by using parameters consistent withthose used
in previous flow models of this area ofCape
Cod, except that numerical dispersion wasmuch
larger than the physical dispersion estimatedin
previous studies. Sorption parameters were fit todata
derived from phosphorus sorption anddesorption
laboratory column experiments. Ratesof organic
carbon decomposition were adjusted tomatch
the location of iron concentrations in ananoxic
iron zone within the sewage plume. The sensitivity of the simulated load of
phosphorustransported to Ashumet Pond was calculated
for avariety of processes and input parameters.Model limitations included largeuncertainties
associated with the loading of thesewage beds,
the flow system, and the chemistryand sorption
characteristics in the aquifer. Theresults
of current model simulations indicate asmall
load of phosphorus transported to AshumetPond
during 1965–85, but this small load wasparticularly
sensitive to model parametersthat specify flow
conditions and the chemicalprocess by which
non-desorbable phosphorus isincorporated in
the sediments. The uncertaintieswere large
enough to make it difficult to determinewhether
loads of phosphorus transported toAshumet Pond
in the 1990s were greater or lessthan loads
during the previous two decades.The model
simulations indicate substantialdischarge of
phosphorus to Ashumet Pond afterabout 1965.
After the period 2000–10 thesimulations indicate
that the load of phosphorustransported to Ashumet
Pond decreasescontinuously, but the load of
phosphorus remainssubstantial for many decades.
The currentsimulations indicate a peak in phosphorus
discharge to Ashumet Pond of about 1,000kilograms
per year during the 1990s; however,comparisons
of simulated phosphorusconcentrations with
measured concentrations in1993 indicate that
the peak in phosphorus loadtransported to Ashumet
Pond may be larger andmoving more quickly in
the model simulationsthan in the aquifer.

The results of the three-dimensional reactive-transport
simulations are consistentwith the loading
history, experimental laboratorydata, and field
measurements. The results of thesimulations
adequately reproduce the spatialdistribution
of phosphorus concentrationsmeasured in 1993,
the magnitude of changes inphosphorus concentration
with time in a profilenear the disposal beds
following cessation ofsewage disposal, the
observed iron zone in thesewage plume, the
approximate flow of treatedsewage effluent
into Ashumet Valley, andlaboratory-column data
for phosphorus sorptionand desorption.

FIGURES

1–4. Maps showing:

1. Location of disposal beds in the study area at the Massachusetts
Military Reservation sewage-treatment plant
near Ashumet Pond, Massachusetts, and extent of the sewageplume
in Ashumet Valley as of 1993–94

2. Areal distribution of maximum dissolved phosphorus concentrations
in ground water near Ashumet Pond, August
to November 1993

3. Altitude of the water table and approximate direction of
ground-water flow in January 1994 near Ashumet
Pond

4. Model grid with 50-meter spacing for reactive-transport
model of the area near Ashumet Pond

5–7. Graphs showing:

5. Results of column experiments of phosphorus sorption and
desorption on Cape Cod sediments and results
of reactive-transport simulations based on fitted parameters

7. Measured and simulated phosphorus concentrations for a column
desorption experiment on a sediment core from
the sewage-contaminated zone of the aquifer

8. Maps showing (A)
measured and (B)
simulated phosphorus concentrations in groundwater
from August to November 1993 near Ashumet Pond

9, 10. Graphs showing:

9. Measured phosphorus concentrations in July 1996 and July
1999 in multilevel sampler S469M1 and simulated
phosphorus concentrations for the vertical set of nodes nearest to the location
of disposal beds 5–8